A valve drive of the precited type is known from GB-A-2 075 118. In this rocker arm, a spring-loaded pressure pin is slidably mounted with a bottom thereof bearing against a valve stem of a gas exchange valve. Above the valve stem, the pressure piston is laterally connected by a coupling means to a spring-loaded and hydraulically displaceable piston. Depending on the end position of this hydraulically displaceable position in each case, the pressure pin is either fixed in place or displaceable so that, upon a pivoting of the rocker arm, either the gas exchange valve is actuated or the pressure pin slides in the rocker arm i.e., the gas exchange valve is not actuated. A drawback of this structure is that, on the one hand, a complicated pressure oil supply comprising a plurality of oil channels is required and on the other hand, the mechanical connection between the pressure pin and the hydraulically displaceable piston is comprised of a plurality of differently configured mechanical components. The manufacture of such a switchable rocker arm is therefore very complicated and accordingly expensive.
In this connection, a switchable valve drive is known from DE 44 11 182 A1 which comprises two transmitting levers with a hydraulically actuatable locking element arranged therebetween. This hydraulically actuatable locking element is supplied with pressure from the oil supply of the valve clearance compensation element arranged in the rocker arm. A drawback of this is that this switchable valve drive is likewise very complicated and expensive to manufacture because two transmitting levers are required.
Finally, DE-GM 93 15 436 also discloses a switchable cam follower in the form of a cup tappet which is composed of two relatively displaceable units whose coupling and uncoupling are assured respectively by a compression spring and by the oil pressure of a hydraulic clearance compensation element. However, this type of switching of two relatively displaceable units is entirely unsuitable for use in a switchable rocker arm.